1. Field of the Invention
The present invention relates to a fuel injection apparatus.
2. Description of Related Art
A fuel injection apparatus, which injects fuel into a combustion chamber of an internal combustion engine, is known.
For example, Japanese Unexamined Patent Publication No. 2006-214317A teaches a fuel injection apparatus, which has a pressure control arrangement. The pressure control arrangement includes a pressure chamber, which applies a pressure against a needle to lift the needle away from injection holes to open the same for injecting fuel when a piston is driven to pressurize the fuel in the pressure chamber. This type of fuel injection apparatus is developed to meet a demand for promoting atomization of fuel by increasing a fuel injection pressure to a higher pressure in a direct fuel injection internal combustion engine, in which the fuel is directly injected in each cylinder.
In the fuel injection apparatus of Japanese Unexamined Patent Publication No. 2006-214317A, an actuator (e.g., a piezoelectric driver), which exerts a large drive force, is used to displace the piston to increase the pressure of the pressure chamber relative to the pressure of the fuel passage, which corresponds to the injection pressure of fuel, so that the injection quantity of fuel is controlled under the high injection pressure condition. However, the above technique of lifting the needle by the pressurized fuel poses the following disadvantage. That is, when the needle is lifted, a volume of the pressure chamber is increased to cause a reduction in the pressure of the pressure chamber. When the pressure of the pressure chamber is reduced, the lifting speed of the needle is immediately, disadvantageously reduced.
In such a case, the lifting speed of the needle right after the opening of the injection holes cannot be maintained. Therefore, the pressure of the pressure chamber cannot be maintained at a required minimum pressure (hereinafter, referred to as a required valve opening pressure), which is required to open the injection holes by lifting the needle against an urging force (e.g., the pressure in the fuel passage) applied toward the injection holes. As a result, the needle is gradually moved downward toward the injection holes to close the injection holes. When this state is encountered, the injection holes are soon closed by the needle to terminate the fuel injection although an injection signal for executing the fuel injection is kept supplied to the actuator. Thus, it is difficult to control the injection quantity of fuel according to the injection signal. Thereby in the previously proposed fuel injection apparatus described above, the stable valve opening operational response may not be obtained.